The invention relates to a method for the optical determination of state variables in the interior of a container for liquidified gases, where light which is emitted from a lighting unit is carried in an optical waveguide to a contact point with the content of the container, where it is partially reflected and the light intensity of the reflected light is measured, and a state variable is determined from this and from the light intensity of the light emerging from the light source.
The container is intended to be a container for low-boiling-point gases—in particular hydrogen—which is carried by a motor vehicle, with the gases being used as fuel. Liquefied low-boiling-point gases are stored in conditions (pressure and temperature) in which the state variables of the liquid phase and of the gaseous phase differ only slightly. Furthermore, the physical and thermal characteristics, in particular of hydrogen, and the heat which is invariably introduced into the interior of the container even with the best insulation leads to an inhomogeneous temperature distribution and mass density distribution in the container.
For safety reasons and in order to make use of the range of one container filling, accurate monitoring of state variables and, furthermore, accurate determination of the amount of fuel still available are required for use in a motor vehicle. The expression state variables means not only the mass density, pressure and temperature but also the phase state (liquid or gaseous). Furthermore, there are certain exacerbating factors which are typical for driving operation, for example the fact that the position of the liquid level is influenced by the inclination of the vehicle and by centrifugal forces.
Conventional level measurement systems have either not satisfied the requirements or have been found to be much too expensive and also unreliable. Electrical measurement methods (capacitive, inductive) require the introduction of electrical lines into the (double-walled) container, which results in heat bridges and a risk of explosion, and, furthermore, they are susceptible to defects and are sensitive to disturbances. Mechanical systems with moving parts are completely impracticable in the low-temperature range.
DE 199 59 279 A1 discloses a method and a device for the optical level measurement in a cryostatic container, in which an optical waveguide touches the container content by means of a sensor surface which is extended in its longitudinal direction. Scattering losses occur in consequence as a function of the refractive index of the container content touching the sensor surface, and these losses increase in proportion to the sensor area and thus to the container content. This allows the liquid level height to be measured. However, because of the poor dependency of the liquid level on the scattering losses and because of the cumulative evaluation of all the scattering losses for the determination of the position of the liquid level, this is not sufficiently accurate. Furthermore, the amount of fuel in the form of gas above the liquid level must also be taken into account for particularly accurate measurement of the available amount of fuel.